I will refer to anything as an “it” if I am unsure of gender. Cats, dogs, squirrels – even humans. Personally, I would prefer people refer to my cat or dog as “it” instead of “she”. About the only time I get touchy about gender is when, during the course of a conversation, somebody continuously refers to my animal as female when I’ve made a point of saying “he” or “his”. Most other people I know don’t get touchy about their animal’s gender – they will make the gender correction and move on with their life.
At this point, if you feel the need to ask why people get “weird”, you have probably made an ass of yourself in such a manner that people feel the need to correct you forcefully. Either that, or you have a really weak definition of the word “weird.”